Hey! I just wanted to chime in here and mention that this seems to be a Lemmy issue. I was able to reproduce this on lemmy.world by following your exact set of steps (create a comment, edit it, don't change anything, save it). Lemmy marks the comment as edited.
In Thunder, we just retrieve the edited state from Lemmy, so I'm not sure we can do anything about this case, but you might want to submit a bug report here: https://github.com/LemmyNet/lemmy